Assessing the carbon compositions and sources of mangrove peat in a tropical mangrove forest on Pohnpei Island, Federated States of Micronesia

Abstract

To evaluate organic carbon (OC) accumulation processes in peat deposits in tropical mangrove ecosystems, solid-state 13C cross-polarization and magic angle spinning nuclear magnetic resonance signals were measured to determine the OC compositions of decomposed leaves and roots. These account for a large majority of mangrove litters and mangrove peat in a coral reef-type Rhizophora forest on Pohnpei Island, Federated States of Micronesia. Radiocarbon dating was also used to understand the sources of peat deposits at each depth. The mass loss rate of mangrove leaves during 1-year litterbag incubation was much higher than that of roots. These mass loss rates are expected to be affected by the varying chemical characteristics of leaves and roots and the different aerobic/hydrological conditions present in the two litter types during decomposition. The decomposability of individual OC components also varied markedly between leaf and root litters. Significant increases in aryl-C/O-alkyl-C and aliphatic-C/O-alkyl-C ratios and minor increases in the aryl-C/aliphatic-C ratio during leaf decomposition implied that O-alkyl-C was more labile than aryl- and aliphatic-C and that aliphatic-C was also slightly more decomposable than aryl-C, but not significantly so, in leaf litters on the forest floor. Regarding roots, a stable aryl-C/O-alkyl-C ratio during decomposition suggested that aryl- and O-alkyl-C components did not differ greatly in decomposability in the peat deposit, while the minor increase in the aliphatic-/O-alkyl-C ratio and the substantial decrease in the aryl-/aliphatic-C ratio with decomposition implied that aliphatic-C was more recalcitrant than aryl- and O-alkyl-C in the peat. The OC compositional properties were quite homogenous throughout the peat profile, and 14C dating mostly indicated modern, which suggest that large amounts of mangrove roots penetrate to at least 80cm depth. These findings provide quantitative and qualitative insights into the potential importance of very high production of mangrove fine roots for OC accumulation in peat in tropical mangrove ecosystems.